These proposed studies will provide presently unavailable ultrastructural, histochemical and cytochemical data regarding the mechanisms controlling fusion between localized regions of the foregut endoderm and overlying ectoderm during early craniofacial development in vertebrate embryos. A comparative histological and ultrastructural study of oral (buccopharyngeal) membrane and closing plate formation will be conducted in representative amphibian, avian and mammalian embryos to determine whether similar ultrastructural features of the cells and intervening extracellular matrix characterize these examples of epithelial fusion throughout the vertebrates. The eltrastructure of cells in those areas of fusion which "disappear" will be compared to those in which the fusion lamina persists and no opening is created. Labelling with 3H-glucosamine, 35S and 3H-fucose and subsequent autoradiography wil be used to determine whether differences in uptake and/or secretion of these precursors are exhibited by cells of the presumptive fusion region relative to adjacent nonfusion areas, and sectins will be stained with Alcian blue to localize sulfated and non-sulfated matrical components in fusion and nonfusion regions. Uptake of H-thymidine and autoradiography will be used to determine: 1) whether epithelial cells in regions of presumptive fusion differ in their rate of DNA synthesis prior to contact when compared to adjacent, nonfusing regions, and 2) whether cells in areas of fusion which "breakdown" differ in pre-fusion DNA synthesis from cells in those regions of fusion which persist. Intracellular localization of lytic enzyme activity within the cells of the oral membrane and closing plates of Xenopus laevis, chick and hamster embryos will be studied by standard histochemical and cytochemical methods.